Transient Overvoltages in a Railway System during ...

Transient Overvoltages in a Railway System during Braking Mario A. Suárez

Jorge W. González

Israel Celis

Electrical Studies H-MV ltda. Medellin, Colombia [email protected]

Electrical Engineering Faculty Universidad Pontificia Bolivariana Medellín, Colombia [email protected]

Customer Mobilization Metro de Medellín Ltda. Medellín, Colombia [email protected]

Abstract—This paper presents an overvoltage analysis in the dc catenary of The Metro de Medellin railway system. The main objective is to study the sources of damages of motors in the trains. It was achieved a detailed study beginning with the direct measurement of variables and then the construction of a digital model in PSCAD-EMTDC. The simulations indicated a high incidence of braking overvoltages imposed on catenaries and insulations during regeneration. Literature and international standards on traction systems, concerning experiences and standardization of voltages, are analyzed. Keywords-railways; switching overvoltages; regeneration; electromagnetic simulation.

I.

braking

INTRODUCTION

The Metro de Medellin is a large capacity railway system that crosses the metropolitan region of Medellin. This system has two lines, A and B. Line A is 23.2 km in length. Line B is 5.6 km in length. The system has rectifying (ac/dc) stations and passenger stations. The Metro de Medellin also offers 3 overhead lines with Metrocable technology. The Metro de Medellin has reported the repeated damages in the traction and compresors motors located in the trains. According to basic measures in the faulted motors, it is suggested that overvoltages could have been the source of damage. In this paper, it is exposed what was done in terms of modeling, simulation and analysis of possible sources of surges on the railway network and that could affect the motors. The modeling of railway systems to study switching overvoltages has been a challenge from the point of view of integrating ac and dc systems, the trains with their controls and the mechanical components. In [1, 2-4] it is mentioned about the lack of models suitable to study switching overvoltages in railway systems. A special topic is the mechanical behavior, nevertheless not much has been done. In [5] a model is developed for the electrical and mechanical torques. Analyses range from the engine and into contact with the rail wheels, involving axles, bogies and mechanical boxes. Nevertheles, the studies do not involve the regenerative braking state and generated overvoltages. Reference [6] studies the switching This work was supported by H-MV Ingenieros Ltda. Medellin, Colombia; Metro de Medellin Ltda., Medellin, Colombia and Universidad Pontificia Bolivariana, Medellin, Colombia.

surges during regenerative braking. However, it is assumed that there are failures in the regenerative braking system in order to create the surges. Reference [2] indicates that there is not much information in the literature regarding switching overvoltages in systems such as trains. In that reference, overvoltage measurements were made and classified for a real train system. In [3] Induced voltages are analyzed in the dc side of a mass system for switching of a capacitor in an ac nearby substation; mitigation mechanisms are proposed. In [4] it is mentioned the lack of standards for application of dc surge arresters, and the lack of literature on these devices. Guidelines for the development of standards are proposed. The reference also indicates a lack of measurement data on surges in dc systems. In [7] it is presented a study to determine resonant voltage level generated within a specific railway system. Reference [8] presented analysis of lightning surges on the lines of electric traction systems using PSCAD-EMTDC. In this paper, in sections II and III, are described basics on dc series motors, and on the control system with special interest on the braking stage. In section IV, the paper analyzes the standards on traction systems in terms of levels and standard voltage ranges. Records of measured variables of interest, with special attention to the voltages are analyzed in section V. In section VI, it is presented a simulation model in PSCAD– EMTDC. The model is accompanied by pictures showing the correlation with the specified operation on the Metro and their relationship to cases reported in the literature. With this model, simulations are achieved to study the influence of certain critical parameters and the overvoltages produced during regenerative braking. Finally, some conclusions and recommendations to avoid surges are presented. II.

BASIC CONCEPTS ON THE DC SERIES MOTOR

The study of the present work is highly focused on the train motors and on the interactions of the control system. The starting and stopping of motors will be of special interest. This section reviews the basic theory of dc series motor and control system performance.

The dc motor is a relative robust motor. Its field winding wire is thick and has relatively few turns, which are connected in series with the armature circuit. Armature current, line and field are the same, see Fig. 1 [9].

c is also constant of motor construction parameters. Then,

Equation (1) is for the terminal voltage of the dc series motor.

The motor torque is proportional to the square of the armature current. The series motor is the dc motor that produces more torque per amperage. However, these motors have a poor speed regulation. From the last expressions, (6) is obtained.


        

(1)

τ   

Where EA, IA, RA are respectively the internal voltage, current and resistance of the armature; RS stands for the field resistance.

(2)

K, φ, ω are respectively a constant of motor construction parameters, the flux and the speed. The flux is directly proportional to the armature current, at least until reaching saturation. An increase in load, equivalent to an increase in flux, causes a decrease in speed and the result is a drastic decrease in the torque-speed characteristic. See Fig. 2. The induction torque is given by (3): τ  KφI

(3)

Until saturation, the flux is directly proportional to the inductor current: φ  





√  !"

#

$% &$' 

(6)

Unsaturated, the series motor has a speed that varies with the inverse of the square root of the torque.

The internal voltage is generated according to (2).   φω

ω

(5)

(4)

III.

THE CONTROL SYSTEM AND POWER NETWORK

A. Control system [10, 11]. The control system is in charge of the operation of trains, including the starting and braking states. Though the system is microprocessor based, it follows the orders from the train driver. This control system has also protection functions. The control system and interactions with the power network is reviewed in Fig. 3. During braking the resistances are connected with decreasing speed. The mechanical stopping brake is required in the area of self des-excitation of the motors (pneumatic brake). The electrical brake is known as dynamic or electrodynamic braking, this stage allows the regeneration, returning energy to the network. Regeneration is possible because during braking it is connected a pre-excitation, for a maximum of 3 s, in the series winding of one of the motors. The braking is preferably electrodynamic, it is automatically activated once the driver returns the lever driving back. In this paper the “electrodynamic braking” will be referred just as “braking”. When regenerative braking is no longer possible, it is required the pneumatic brake.

Figure 1. Variables and parameters of the dc series motor

Figure 2. Speed – Torque curve of the dc series motor

Figure 3. Basic scheme of power and control system of trains [10, 11].

During the braking the corresponding thyristor is turned on. The energy is dissipated in the braking resistor in the absence of any receiver in the network for this energy returned, for instance a train during traction; and if the network capacitor voltage has exceeded a permissible limit value. The theoretical value for the braking control network has been set to 1800 V. In the case of rapid changes in the network capacitor voltage, the braking thyristor is forced to fire when it has exceeded 1950 V. The chopper controls the motor in both traction and braking. The chopper operates continuously with 250 Hz pulses. The energy generated by braking is returned to the network depending on its ability of admission, or is transformed into heat in the dissipation resistance. If the network does not accept the power of regeneration by braking, the current through the free wheel diode is directed to charge the capacitor increasing its voltage. This situation is the most critical, as it will produce the greatest stress on the network. If the voltage value exceeds a pre-set maximum value (1800 V or 1950 V), the control will activate the braking thyristor, causing braking by dissipating (braking) resistor. This process of checking if the network accepts the energy offered is repeated in each period of the frequency of service, i.e., every 4 ms. The capacitor network is also used as a low pass filter. Working alone eliminates the peak voltage that occurs in the chopper. Working with network inductance forms a filter unit that smooths the current that is fed to the network. The limit voltage of the capacitor is 2300 V. The pre-induction exciter is applied so that the electrodynamic braking can act as the inductor needs an initial magnetic remanence. B. Some power network components [10, 12] The rated voltage of the catenary is 1500 V dc. The Metal Oxide surge arrester is 2 kV, 10 kA. The motors are dc series and have a rated voltage of 1420/2 V, 300 A, 205 kW, maximum speed 3500 rpm. Fig. 4 shows main power components.

IV. STANDARD IEC 60850 The Standard IEC 60850 [1] is related to supply voltages of traction systems for railway applications. Concerning the current work, the standard specifies the voltages for a 1500 V system as the Metro de Medellín, as shown in Table I. This standard has to do with long-term surges. The specified values are averages of the dc voltage. TABLE I.

RATED VOLTAGE AND AVERAGE PERMISSIBLE LIMITS AND DURATION FOR A SYSTEM OF 1500 VDC.

Minimum non permanent voltage Umin2 [V] 1000

Minimum permanent voltage Umin1 [V]

Rated voltage

1000

1500

Un [V]

Maximum permanent voltage Umax1 en [V] 1800

Maximum non permanent voltage Umax2 [V] 1950

Adapted from IEC 60850.

The following requirements have to be met: -The duration of voltages between 1800 V and 1950 V cannot exceed 5 minutes. During normal operation, the voltage should be in the range 1000 V ≤ U ≤ 1950 V. -At no load, the voltage at the substation cannot exceed 1950 V, provided that in the presence of a train, they meet the voltages of Table I. During abnormal operating conditions, not damages must be submitted at 1000 V. -If during operation the system reaches voltages between 1800 V and 1950 V, they must be followed by a voltage threshold below or equal to 1800 V for a period not specified. Voltages between 1800 V and 1950 V can only be achieved due to non-permanent operating conditions such as regenerative braking. Annex A of the standard presents three duration zones according to the allowable limit stresses. These are shown in Fig. 5 for a 1500 V nominal voltage. V.

REVIEW OF RECORDS AND MEASUREMENTS

The Metro has measured electric variables in normal service. After analyzing the records, it has been found that Line B is the most influenced by surge voltages, having high value and repetitive during time. Line A poorly exposed some critical values of stress. For this reason, more emphasis is made in the surges of Line B.

Figure 4. Components of the Metro de Medellin Network

Figure 5. Maximum value of the voltage U in accordance with the duration (adapted from Annex A of IEC 60 850, 2007-2).

For line B, there are records showing surges entering the range between 1800 V and 1950 V for approximately 30 s. This is related to a regenerative braking situation, but could not directly classify as a malfunction according to the standard IEC 60850. The standard allows 5 minutes for such surges in the indicated range. However, records can confirm that these incursions into the interval between 1800 V and 1950 V, D critical area according to the IEC60850 standard, are repeated frequently in Line B, with minimum time intervals of approximately two minutes. A more important point is that higher voltage surges appear at 1950 V, and repetitive as well. As an example, it was particularly analyzed a record of successive surges with values of 1976.8, 1966.4, 1954.4, 1968, 1977.6 and 1980 V. The allowed duration of voltages can be calculated with the equation in Annex A of standard IEC60850. For example, for 1966.4 V substituting in: U = 1950t-0.0676 yields t = 0.8835 s or 883.5 ms. Nevertheless, the approximate length of this surge was 200 ms. It could be said that there is no problem with respect to the standard. The problem would be if the design of the Metro de Medellin is not made based on IEC60850 and the surges with such repetition in time are sufficient to aging or damaging the insulation of motors. The appearance of the previous voltage surges could be obtained by the sum of surges from some trains when there is not enough traction requirement from other trains in the section of catenary. The latter for control issues of capacitor voltage as well as the likely depreciation in value of capacitance; for resistance problems of dissipation in terms of their devaluation and for parameters degradation. It could also be thought that the surges can be obtained by the regeneration of several trains stopping in the same section of catenary, or even originated from several carriages of a train. Several of these suppositions are part of the digital simulations of the coming sections. The records analyzed for Line B were found to have higher density voltage surges after about 4, 5, 6 or 7 hours of commercial service started in the Metro, that is, after 8:30 a.m., or 9:30 a.m., etc., which coincides with the "valley hours" service. A relevant explanation is that during these times there are fewer trains pulling or being recipients of regenerative braking energy from other trains. This phenomenon will also be verified in the digital simulations of the next section. As indicated at the beginning of this section, Line A is less influenced by surges, there is less intensity of high values of voltage and less repetitive. There were not many records exceeding 1800 V in Line A, which occurs with regenerative braking. No value exceeded 1950 V.

basic and generic model meeting the most critical conditions that could lead to surge problems. The model has been developed for the worst operating condition that could produce the highest surge switching. In accordance with the discussion in the previous section, the most critical situation in terms of surges is during regenerative braking when the network does not accept the power of regeneration. The current through the free wheel diode will be aimed at increasing the charge of the capacitor. During braking, energy is dissipated in braking resistance when there is no receiver in the network for the energy returned, and the network capacitor voltage has exceeded an allowable limit mentioned. The model in PSCAD corresponded to a system of a section of catenary fed from the ends of two equivalent ac networks, that correspond to the selected substations, Acevedo and Universidad. Both substations are tied through a dc catenary. The model consists of two end rectifiers with filters; everything has been built in accordance with the actual parameters of the system. There is a train at Acevedo station, which will be simulated as much in its starting state, steady state traction and as braking. In the middle of the dc line will be located another train for joint analysis of operation. The model was constructed using an equivalent motor for each pair in series. The Metal Oxide Varistor is modeled with the manufacturer's catalog and test reports using the I-V curve for steady state and surge type switching [14]. The Chopper was modeled according to specifications, i.e., frequency of 250 Hz. The mechanical system has a basic modeling, obtained by trial and error technique, which provides a conventional operation of the motors, demanding an average of 300 A. The mechanical model solves the differential equation describing the dynamics of a motor as in (7). ()  *

+, +-

 .ω) + ()

(7)

Where: Tm: load torque [N-m] Te: electrical torque [N-m] J: inertia [kg-m2] D: damping coefficient [N-m-s / rad] ω = rotor speed [rad / s]

VI. DIGITAL MODEL AND SIMULATIONS This section describes how it was developed a model for digital simulation using PSCAD-EMTDC [13] for the Metro de Medellin railway system. It will allow studying the generation of surges during operation. A base case simulation is presented. A. Digital model implemented Due to the diversity of section lengths of the catenary system, the equivalent of feeder circuits, the slopes of the track, the direction of traffic, the number of wagons, number of passengers, trains and concurrency, there should be created a

The modeling will be basic. It will aim to expose the phenomenology and typical behaviors of the system when it is excited in different ways. The aim will be to analyze the cause of surges and the general phenomenology. The detailed mechanical modeling for a drive system is now even global research material; little has been done on this subject, which is beyond the scope of this project. In [5] it is mentioned the need for further research on the mechanical models. In that reference, equations of electrical and mechanical torques are developed, the analysis go from the motor to the contact with the rail wheels, involving axles, bogies and mechanical boxes.

An interesting assumption that successfully shows the reference is that the weight of the whole train is evenly distributed between the bogies, allowing obtaining a simplified model based on a bogie. However, the application of that research is the analysis of steady-state conditions, starting stage and three-phase short circuit. There were no references to braking, main issue of interest in this project.

time of 50 s, and time steps of 5 and 100 µs. Figs. 7 to 9 depict the latter. In Fig. 8 appears the network voltage Ea1. It can be seen that before braking, the voltage is stabilized in an appropriate value with an average of 1500 V. At 25 s, during braking, there is a surge, then energy stabilization. Later it will be shown that this energy is consumed by the dissipative resistance, and finally stabilized at approximately 1625 V.

The line parameters correspond to the section Acevedo University (4.1 km, 0.2 ohm, 5.6 mH). The line was modeled using distributed parameters. See Fig. 6.

In Fig. 9 is the armature current of the motor and power dissipation of the resistance (“Ifrenado”). In stabilizing, the armature current reaches an average of 300 A. The maximum braking current is 690 A.

B. Base case simulated According to the basic rules of operation, in the case of regenerative braking without power demanded from another train, the base case was simulated as follows. The Chopper initialization and connection of mains contactor was at t = 1 s. The Chopper was initialized, increasing gradually its phase, through a ramp leading up until t = 15 s. The motor contactors were connected at t = 1.1 s. Once stabilized, the motor braking was ordered at t = 25 s. The braking order immediately connected the pre-excitation of the field. The connection was activated for 3 s. Then, 3 s later, resistors were connected for field weakening. Since an overvoltage occurred during regenerative braking, the dissipative resistance was connected when the capacitor voltage reached 1950 V. In this base case there was actually an overvoltage since the remote train was disconnected. The simulation was performed for a maximum

It may be noted, that despite the controlling action of the dissipating resistance, the network voltage reached a maximum of 2548 V, and there was a second high peak of 2428 V. It was found that the surge arrester conducted during these two surges. C. Other simulations This section presents simulations as guidelines and assumptions of the influence and sensitivity of parameters, variables and aspects of the traction control system, which could create damaging surges for electrical insulation. The model was designed to explain the tendency of the general phenomenology of parameters and logical variables. UNIVERSIDAD

5.6e-012 [H] Com. Bus

ACEVEDO

AM GM

0.002 [ohm]

E_ACE

TRAIN

B1K53

7 .5 [o h m ]

B1K53 +

3 9 0 0 .0 [o h m ]

If B1K1

V1

KB alfa 1.0

0.005 [H]

+

B1K2_3_V1

2 8 0 0 0 .0 [o h m ]

7 5 6 .0 [u F]

3 9 0 0 .0 [o h m ]

Rdebil

AO

Ia Em1

w wm

+ -

Te Te

B1K2_3_V1

Chopper Chopper

V4 EU T

V2

1 .3 4 [o h m ]

+ V3

6 Pulse Bridge

0 .0 0 0 1 [o h m ]

0.0012 [H]

3 9 0 0 .0 [o h m ]

1 6 .0 [u F]

3 9 0 0 .0 [o h m ]

AM GM

1 6 .0 [u F]

1 0 0 0 0 0 0 .0 [o h m ]

Com. Bus

KB alfa 1.0

1 .0 [o h m ]

#3

#1

1 .0 [o h m ]

6 Pulse Bridge

B1K51

AO #2

B1K52

Ea1

1.5 [ohm]

Rdebil

Freno_Disip

+ Em2

I_B4 B1K4

Figure 6. PSCAD-EMTDC model between Acevedo and Universidad stations.

+ -

w wm Te Te1

Chopper

1.50

Ton

0.0 t(s)

38.520 38.530 38.540 38.550 38.560 38.570 38.580 38.590 38.600

... ... ...

Figure 7. Chopper signal. 2.60

Ea1

Em1

Em2

2.40 2.20 2.00 V[kV]

1.80

In Fig. 11 is the network voltage for the train not braking. In Figure 10, it is noted that the maximum voltage peak on the train that brakes is 1810 V, which is lower than the case of regeneration without receiver of Figure 8, in which 2548 V was obtained. This confirms the utilization of regenerated energy. For the remote train, in Figure 11, there is an increase in the voltage wave at a value of 1900 V, probably by reflection of the transient traveling waves of voltage. Fig. 12 shows armature current Ia of the train that brakes, Iab of a train pulling and the dissipative braking current “Ifrenado” during starting up, stabilizing and braking of a train with receiver. The dissipation resistance did not act because the voltage has not reached 1950 V. In Fig. 13 is presented the current in the surge arrester (DPS) of the train braking, the performance of DPS was zero as expected by the low voltage.

1.60

1.90 1.40

1.80 1.20

1.70 1.00 24.990

25.000

25.010

25.020

25.030

25.040

... ...

Figure 8. Network voltage during start up, braking and regeneration without receiver.

1.60 1.50

V[kV]

t(s)

1.40 1.30 1.20

800

Ia

1.10

ifrenado

1.00 t(s)

Armature Current (A)

600

24.9900

24.9950

25.0000

25.0050

25.0100

25.0150

25.0200

... ... ...

400

Figure 11. Network voltage at remote train not braking. 200 0

Ia

700

ifrenado

Iab

600

-200

500 24.0

25.0

26.0

27.0

28.0

29.0

30.0

31.0

32.0

... ... ...

Figure 9. Armature current and braking (“frenado”) via the dissipative braking resistor.

1) Surges in motors for regeneration from other motors It was assumed that both trains started up at the same time. The first train started braking after 25 s. The second will be pulling and will be receiving part of the regenerated energy. The trains are in stations separated 2.05 km. In Fig. 10 appears the detail of the network voltage at the train that breaks.

400

Armature Current (A)

-400 t (s)

300 200 100 0 -100 -200 -300

t (s)

0

40

50

... ... ...

0.80m

1.70

0.75m

1.60

0.70m

1.50

I [A]

V[kV]

30

I_DPS

1.80

1.40

0.65m

1.30

0.60m

1.20

0.55m

1.10

0.50m

1.00 24.9900

20

Figure 12. Train currents in motors. One braking..

1.90

t(s)

10

24.9950

25.0000

25.0050

25.0100

25.0150

25.0200

... ... ...

0.45m t(s)

24.9800 24.9850 24.9900 24.9950 25.0000 25.0050 25.0100 25.0150 25.0200

Figure 10. Network voltage at train braking. Figure 13. Current of the surge arrester in a train.

... ... ...

2) Superposition of regenerative braking overvoltages during trains concurrency Both trains start up at the same time. It is assumed that the first train brakes after 25 s. The second brakes 1 s later. In this situation, the second train uses the regeneration of the first before braking. When the second brakes, it is done without receiver, there appear two consecutive surges different in magnitude. At the time of braking the trains are in stations separated 2.05 km. See Fig 14 in which it is detailed Ea1 voltage and the appearance of two voltage surges by the brake. 3) Reduction of brake adjustment by dissipative resistance. This case is made with a single train braking, in order to see the control of the maximum voltage when the voltage setting for activating the braking resistor is reduced to 1800 V. Fig. 15 shows the detail of the voltage for the base case with a setting of 1950 V. Fig. 16 shows the setting for 1800 V. It may be noted the benefit of reducing the pickup setting dissipation resistance at 1800 V. The peak voltage is reduced to 2452 V in contrast to 2548 V obtained with the setting of 1950 V of base case. The latter means a reduction of 3.8% in overvoltage. Ea1

2.40

Em1

Em2

I_DPS

2.20 2.00

V[kV]

1.80 1.60 1.40 1.20 1.00 0.80 t(s)

23.0

24.0

25.0

26.0

27.0

28.0

29.0

... ... ...

Figure 14. Voltage Ea1 during braking of two trains, one at 25 s and the other at 26 s. 2.60

Ea1

Em1

On the other hand, it was noted that in both cases the power dissipation varied very little. For the setting of 1950 V the conduction was done until 29.55 s. For the setting of 1800 V conduction extended to 29.93 s (380 ms difference). It was also noted that the first peak of conduction current through the resistance of dissipation is slightly larger in magnitude for the setting of 1800 V, which was expected since the resistance will have to dissipate more energy. Figure 17 shows the surge arrester DPS conduction when the setting is 1800 V. It may be noted that conduction is lower than for the dissipative resistance setting of 1950 V as shown in Fig 18. 4) Locking regeneration control by dissipation resistance The effect of blocking the regeneration control by dissipating resistance produced a maximum voltage peak of 2675 V. This has been the biggest surge peak obtained. It was found that the surge arrester conducted practically during all the braking, which could be risky for its integrity. 5) Modification of breaking by dissipation resistance To observe the sensitivity of overvoltages by regenerative braking without a receiver, depending on the dissipation resistance, two simulations are carried out. The first is for a reduction in dissipation from the installed 1.5 ohm to 0.5 ohm. In the second case it was increased to 5 ohm. In the first case it was observed that the variation of the peak voltage is very low, the voltage reached 2560 V, versus 2548 V obtained in the base case (1.5 ohm). In Fig. 19 is seen that the overcurrent through the resistor dissipation would be too high. For the dissipative resistance of 5 ohms the reduction of the voltage peak is not significant with respect to the base case. The voltage reached 2500 V, versus 2548 V in the base case. It was found that the current in the dissipation resistance decreased against the other cases, because of the high dissipation resistance value.

Em2

12.0

2.40

I_DPS

10.0

2.20

8.0 I [A]

V[kV]

2.00 1.80

4.0

1.60

2.0

1.40

0.0

1.20

t(s)

1.00 t(s)

24.990

25.000

25.010

25.020

25.030

25.040

... ... ...

Figure 15. Overvoltage Ea1 during regenerative braking without receiver, disipative resistor control in 1950 V - base case. 2.60

6.0

Ea1

Em1

Em2

... ... ...

24.9940 24.9960 24.9980 25.0000 25.0020 25.0040 25.0060 25.0080 25.0100 25.0120

Figure 17. Current in the surge arrester -DPS- during braking with no receiver, for the control of dissipative resistance in 1800 V.

I_DPS

30.0

2.40

I_DPS

25.0

2.20

20.0 I [A]

V[kV]

2.00 1.80

10.0

1.40

5.0

1.20

0.0 t(s)

1.00 t(s)

15.0

1.60

24.990

25.000

25.010

25.020

25.030

25.040

... ... ...

Figure 16. Overvoltage Ea1 during regenerative braking without receiver, disipative resistor control in 1800 V.

24.9925 24.9950 24.9975 25.0000 25.0025 25.0050 25.0075 25.0100 25.0125 25.0150

... ... ...

Figure 18. Current in the surge arrester -DPS- during braking with no receiver, for the control of dissipative resistance in 1950 V.

6.0k

Ia

ifrenado

motors and typify the surge arresters behavior, subject not clearly treated in known literature. Other subject is to evaluate whether the exiting motors have been submitted to an accelerated aging during normal operation. Finally, due to the strength of Line A and length (with more trains pulling power) it should be interesting to analyze its interconnection to Line B for controlling the voltage.

Iab

5.0k 4.0k Current (A)

3.0k 2.0k 1.0k

ACKNOWLEDGMENT

0.0

The authors thank the contributions of Jorge Alberto Vélez at H-MV, Claudia Cardona at Metro de Medellín; and Hugo Cardona, Idi Isaac, Gabriel López and Andrés Díez from UPB.

-1.0k -2.0k t (s)

24.50

25.00

25.50

26.00

26.50

27.00

27.50

28.00

... ... ...

Figure 19. Current through dissipative resistance if its value were reduced to 0.5 ohm with respect to the base case of 1.5 ohm.

VII. CONCLUSIONS Regenerative braking can cause switching overvoltages in the catenaries of a railway system. These surges can affect the insulation of the equipment in the trains. The literature expresses a general worry for the lack of switching surges studies and classifications in dc systems. Modelation and digital simulations achieved in this work supported the latter. Particularly, it was found that the shortest line, Line B, presented repetitive overvoltages during operation, in contrast to Line A which worked within ranges of standard IEC60850. It has to be highlighted that maybe for the age of the project; it was not designed according to that standard. The overvoltages in Line B where more accentuated during low demand hours. This agrees with the fact of less quantity of trains demanding the excesses in power regenerated during braking. As part of simulations sensitivities with the digital model, It was found that the maximum overvoltage would result for a control block of dissipation resistance. In this case the surge arrester conducts practically all the braking, which could be risky for its integrity. The reduction in the value of the dissipating resistance was not significant to reduce voltage surges, but in opposition to good results, it was critical the high value of current through this resistance. So this strategy should not be recommended in the search of avoiding voltage surges. One strategy that did show good expectancy was to diminish the voltage setting of activation for connecting the dissipating resistance; the simulations supported this. It is also a concern, to verify the capacitance value of the network capacitor, due to its tasks, a change in this parameter would be critical in favor of producing overvoltages. A further study on insulation coordination for the equipment in the trains should be advanced in the future. One critical aspect is to evaluate the insulation capability of the

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